Such a method is known in particular from German patent application DE-29 32 196. In that application, the preforms to be assembled typically have a length and a diameter of 300 mm and 10 mm, respectively. The heating means used is a blow torch, or a resistance oven, or indeed a laser. The cylindrical preforms are rotated about their axis to ensure that heating is uniform around the periphery of each end.
Nowadays, the preforms used have a diameter that lies typically in the range 100 mm to 170 mm, so as to increase the length of optical fiber that can be drawn therefrom.
Assembling such preforms together requires heating power that is much greater than that required for preforms that are 10 mm in diameter, given that the volume of material that needs to be softened varies with the square of the diameter. In addition, heat must be delivered in such a manner that the material softens in substantially uniform manner throughout the heated volume at each end; this is particularly critical since the periphery and the core of the preform do not have the same melting temperature, with the periphery of the preform generally containing no doping elements, whereas the core does contain doping elements.